Digital computing machines commonly transfer data between themselves over voice-grade telephone lines by using a data communications adapter commonly known as a "modem." A modem both converts digital data from a digital computing machine into an analog signal suitable for transmission over voice-grade telephone lines, and also converts a received analog signal into digital data for use by a digital computing machine.
According to several common standards for modems, there are two modes of operation, called originate and answer. Each mode has a specified carrier frequency thus allowing full duplex communications between two digital computing machines. Many medium speed, for example 1200 bits per second, modems employ four phase differential phase shift keying (PSK). Under this standard two bits (a "dibit") of data are encoded for each baud time and transmitted by quadrature differential phase shift keyed modulation of the carrier. The data, two bits, is recovered by measuring the differential phase shift between the carrier signal for the current baud and the carrier signal for the immediately preceding baud. Since there are 4 possible phase values, each separated by 90.degree. from adjacent phase values, this modulation scheme is also called Quadrature differential PSK or QDPSK.
A typical method of recovering data is to duplicate the incoming signal in two parallel signal paths, and simultaneously mix the signals in the two paths with two separate reference signals, both at the carrier frequency but displaced from each other by ninety degrees in two separate synchronous detectors. This provides two detected signals. The duty cycles of both detected signals are then simultaneously measured. The two duty cycles for the present baud are compared to the two duty cycles for the immediately preceding baud and the results of this comparison provide information from which the differential phase shift between successive bauds is determined. The differential phase shift is then decoded to provide the current dibit of data.
Although this method is used in many systems and performs satisfactorily, it suffers from the disadvantage of requiring two duplicate processing streams for the incoming signal thereby increasing the number of components, the power required, the heat to be dissipated, and the cost, and decreasing the system reliability or mean time between component failures. Also, the requirement for simultaneously measuring duty cycles in two parallel signal paths has heretofore made the use of a single microprocessor to accomplish demodulation impractical. Therefore, QDPSK demodulation has heretofore been accomplished with external circuits in microprocessor based modems. There is therefore a need for a PSK demodulator which recombines the signal paths prior to measurement of the duty cycles so that a single microprocessor can be used to recover the data encoded in a PSK signal.
Furthermore, due to the limited bandwidth of voice-grade telephones and to the modulation techniques used in some modems, the ideal, abrupt change in phase cannot be transmitted with perfect fidelity, thus distorting the first portion of a transmitted baud and creating a small but measurable probability that the recovered data will contain an error.
There is therefore a need for a PSK demodulator which does not utilize the first portion of a transmitted baud to recover the data.
There is therefore also a need for a baud clock recovery circuit which generates a recovered baud clock which is offset in phase from the transmitting baud clock so that the PSK demodulator is unaffected by distortion in the first portion of a transmitted baud.